Though the Franks have been planning for a move to the Annex for a while, and have been transferring belongings and supplies there in small quanitities, they have not been able to complete their preparations when the notice of Margot's deportation arrives.
The Franks and the others know that they will be in the Annex indefinitely, so they must bring us much as they can with them to last for the duration. They wear as much as they can, in anticipation that this is all the clothes they will have. The purchase of additional clothes will be possible through Miep, but requiring frequent trips to buy clothes will arouse suspicion.
Carrying luggage for clothes will also cause suspicion from the Nazis. So many people were trying to escape being deported to the concentration camps that they were constantly on guard of anyone who is carrying luggage. Being stopped would require presentation of papers, which would then identify Margot as one who is supposed to be deported. Thus the Franks are trying to avoid as much attention as possible.